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Map Sensor

3315 Views 5 Replies 2 Participants Last post by  gtx2000
I mean i cant test this thing at all no one sells a MAP SENSOR tester no more. Has anyone heard of these hanging but will not show a code in the computer i got a 87 Vulcan and i got a popping noise fromt eh engine it is backfiring. I know it is not the common things like plugs wires timing TFI or the Pickup i got good compression and a new coolent temp sensor all fluids full replaced the knock sensor and also cleaned and replced the upper and lower manifold gaskets and rebuilt the fuel injectors with a new fuel filter and air filter new PCV and an oil change i oly got the 02 Sensor and the MAP Sensor to replace im running out of ideas ANYone?
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I also scanned it and there is no codes and only backfires when getting off the ramps of freeways and hiways at a stop when i put it in park it goes away even if i feather the gas it seems to disapper. It still happens on side streets but not as often i need to get this fixed this car is in mint shape and is my grandmothers and i dont want it breaking down on her ive got money into this and no luck on getting it fixed
A MAP sensor works by taking the manifold pressure (vacuum) that is applied on it, and creating a voltage based on the amount of vacuum. It sends this to the PCM, which then uses this information to fire fuel injectors, etc.

You should be able to test it on the car. You'll need to backprobe the connector with a voltmeter, and get a reading with the key on, engine off. After that, start the motor, and CAREFULLY backprobe another voltage reading. It should change. I don't have the specs on how much it should change, but you should see a change. If you don't, chances are it's bad.

Another thing has come to mind. How many miles are on this car? I've seen stretched timing chains do the same thing, but only after a lot of miles.
Theres 115k on it now. I thought the Map sensor emitted HZ and not voltage i got the specs at work i got mitchell document and specs but it said that i should use the tester. I also am getting a Valve rattle when there is a load on the engine. When im by myself there is no rattle and if their is im going up a hill usually a steep one. the car also searches for a idle when first stated up. I also get small backfiring i can barley hear it i can feel it on the brake pedal and this usually happens when i get off the freeway, When i put it in park it magically goes away. I just had the Transmission done 3 weeks before this starting happening and now im let with the O2 sensor and the map sensor to replace i got 35-40 psi in the fuel system and i got a new fuel filter soo im stumped i think im gonna try to replace the MAP sensor to see what happens.
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The MAP sensor puts out an AC signal whose FREQUENCY is proportional tot he vacuum/pressure applied to the sensor.

Any multimeter that has a frequency counter built in can test the sensor. These meters are not that expensive (about $60 at Sears, for example). Just use a hand operated vacuum/pressure pump to activate the sensor and read the output frequency.

Frequency output for pressure readings would vary from 122 Hz @ 17 in-Hg to 162 Hz at 31 in-Hg.

For vacuum, the range of output should be 80 Hz @ 30 in-Hg to 159 Hz @ 0 in-Hg (30 in-Hg ambient pressure).

Ford has used the same sensor as both a vacuum and as a pressure sensor. The sensor will measure both ways.

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thanks for the info i just got one form ford and installed it and i still got that same rattle under the hood on load but it will not do it all the time wierd i think im jsut going to give up this is the wierdest thing ever
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